This invention relates generally to improved designs of devices used on spacecraft and commonly referred to as thermal straps or cold straps or flexible conductive links (FCLs) for providing thermal (conductive) coupling and structural decoupling between cryogenic components such as a vibrating cooling source and a motion-sensitive element or focal plane array (FPA) having highly critical alignment requirements. The present invention provides means for reducing the stiffness of thermal straps through the use of negative-stiffness mechanisms thereby improving their structural decoupling. In the subsequent discussions, the terms thermal strap, FCL and cold strap are used interchangeably. Also, the combination of negative-stiffness mechanisms with a thermal strap or an FCL or a cold strap will be referred to as a “negative-stiffness thermal strap (NS thermal strap)” or a “negative-stiffness FCL (NSFCL)” or a “negative-stiffness cold strap (NS cold strap).”
A critical tradeoff in the design of the thermal strap is maximizing the thermal conductance, which improves the overall performance of the thermal strap, and maximizing the structural decoupling which requires minimizing the stiffness. These design factors present conflicting design goals to the spacecraft engineer. It would therefore be beneficial if a thermal strap or other coupling device could attain maximum thermal conductance while at the same time maximizing structural decoupling in order to effectively isolate vibrations from the motion-sensitive equipment. My previous thermal strap invention, Improved Thermal Straps for Spacecraft, U.S. application Ser. No. 13/587,207, filed on Aug. 16, 2012, solves these and other needs.
In my previous thermal strap invention, it was shown that negative-stiffness mechanisms could improve the structural decoupling of a thermal strap or could improve the thermal conductance, or could improve both the structural decoupling and the thermal conductance. In that invention two thermal straps were used in series. The first thermal strap was combined with negative-stiffness mechanisms that removed much or all of the stiffness of the first thermal strap in the axial direction and in directions transverse to the axial direction. The first thermal strap was relatively stiff in tilt, or rotation about any transverse axis. The second thermal strap provided low tilt stiffness and structural decoupling in tilt but did not have the benefit of negative-stiffness mechanisms. However, with the higher thermal conductance that can be achieved in the first thermal strap for the same or lower axial and transverse stiffnesses, the thermal conductance of the second thermal strap can be made lower to allow for a lower tilt stiffness so that the combined thermal straps will provide improved thermal coupling or improved structural decoupling, or both improved thermal coupling and structural decoupling compared with conventional thermal straps.
My prior thermal strap invention relied on mechanisms which can apply negative stiffness to an elastic structure having positive stiffness in order to cancel, or nearly cancel the positive stiffness of the structure. These previous inventions utilized negative-stiffness mechanisms to provide vibration isolation systems capable of supporting an object having weight (an object with mass in a gravitational field) and providing low stiffness and low natural frequencies in both the vertical (gravity) direction and in the lateral or horizontal directions. The low horizontal stiffness and low horizontal natural frequencies were achieved by using the weight of the object to load vertically oriented beam-columns close to their critical buckling loads (the loads at which their lateral stiffness becomes zero). This approach made use of the “beam-column” effect, which refers to the reduction in the bending stiffness of a beam when it is loaded in compression to make the beam behave as a beam-column. It can be shown that the beam-column effect in a vertically oriented beam-column is equivalent to a horizontal spring and a negative-stiffness mechanism, and the magnitude of the negative stiffness increases with an increase in the weight load. The low vertical stiffness and low vertical natural frequency was achieved by using a support spring connected to a negative-stiffness mechanism in the form of horizontally oriented beam-columns which are spring loaded in compression so that the negative stiffness removes much of the stiffness of the support spring and the stiffness of the beam-columns. These vibration isolation systems are used to isolate vibration-sensitive objects from the vertical and horizontal vibrations of a vibrating support, i.e., to reduce the magnitude of the vibrations transmitted from the vibrating support to the object.
These prior vibration isolation systems are described in U.S. Pat. No. 5,530,157, entitled “Vibration Isolation System” issued May 10, 1994, U.S. Pat. No. 5,370,352, entitled “Damped Vibration System” issued Dec. 6, 1994, U.S. Pat. No. 5,178,357, entitled “Vibration Isolation System” issued Jan. 12, 1993, U.S. Pat. No. 5,549,270, entitled “Vibration Isolation System” issued Aug. 27, 1996, U.S. Pat. No. 5,669,594, entitled “Vibration Isolation System” issued Sep. 23, 1997, U.S. Pat. No. 5,833,204, entitled “Radial Flexures, Beam-Columns and Tilt Isolation for a Vibration Isolation System issued Nov. 10, 1998, which are all hereby incorporated by reference in this present application. These vibration isolators exhibit low stiffness, and low fundamental resonant frequencies, high damping to limit resonant responses of the composite system, effective isolation at the higher frequencies, and can provide high isolator internal resonant frequencies.
It would therefore be beneficial if a thermal strap or other coupling device could attain maximum thermal conductance while at the same time maximizing structural decoupling in order to effectively isolate vibrations from the motion-sensitive equipment. It also would be beneficial if tilt stiffness associated with the thermal strap could be reduced through application of negative stiffness for tilt, thereby improving the thermal coupling, It would also be beneficial if means could be shown for reducing parasitic heat transfer in the thermal strap. The present invention solves these and other needs.